Flame monitors currently in use are provided with several methods of assuring continued burner operation despite fluctuations in flame intensity. One method is to provide a time delay after the flame intensity had dropped below a given threshold before de-energizing the flame relay. Then, if the cause of the drop were only a momentary fluctuation of the flame due to smoke or other cause, while the flame is still actually functioning properly, the system will not be shut down. Another method is to establish a given threshold of flame intensity to be reached before indicating a flame-on condition and arranging it so that, thereafter, the flame-on condition will continue to be indicated as long as the intensity of the flame does not fall below a given lower level (referred to as a "hysteresis" type of operation). In this way, a measure of control is provided during start-up by assuring that a flame-on condition will not be indicated until the fluctuations exceed the given high threshold.
Such methods are reasonably adequate for gas and oil flames in which the fluctuations during normal operations are relatively minor. They are not adequate, however, for low caloric fuels, such as pulverized coal and waste fuels used in the process industries, particularly during start-up operations in which the intensity of the flame may fluctuate radically giving an indication of a flame-on condition at a time when the fire box may not be hot enough for burner operation without igniter assistance. What is needed is to prevent the indication of a flame-on condition until the flame intensity has remained at a given value for a preselected period of time. This need for detection of flame intensity duration arises also, for example, when starting up an oil burner in an already operating waste fuel furnace at a time of change-back from waste fuel to fuel oil. The monitor of the oil burner must be arranged so as to avoid falsely indicating a flame-on condition in response to the high intensity, momentary flares from the waste fuel flames in the background. Here again, an extended duration indication of a given flame-on signal level containing for a substantial period is needed to prevent these momentary flares from the waste fuel flame from falsely indicating a flame-on condition for the oil flame.
Accordingly, a primary object of the invention is to provide means in a flame monitoring for sampling a flame and limiting the indication of a flame-on condition on start-up to situations in which the flame intensity has exceeded a given value for a preselected duration. A further object is to provide such a monitor together with means for delaying the indication of a flame-off condition following the detection of normal flame-on conditions. Still another object is to provide means for maintaining the respective durations of delay of the respective flame-on and flame-off indications in a proportionally fixed relationship.